This was added recently, and I don’t see any reason not to allow it, but I thought I’d leave it open to discussion for a few days in case someone has any opposition to it:
http://tickets.musicbrainz.org/browse/STYLE-652
It’s not a particularly common case (at all), but it sounds to me like interesting info to store nevertheless.
Is this meant for inventors or people who made a specific instrument? I can see a clear case for the former (and implementing it would probably be as easy as adding a new relationship). The latter would be interesting as well (for example famous organ builders), but I don’t really see how that would work in the database, because specific builds of an instrument aren’t MusicBrainz objects.
For inventors of specific instruments (and yeah, it’s supposed to just be a relationship). Now that you mention it, it would be awesome to have a list of Cavaillé-Coll organs, but yeah, that’s harder and out of the scope of this one 
InstrumentBrainz!
Or possibly ToolBrainz to be broader (and because of the 20 character minimum for posts 
).
I’ve added this now. I’m not super happy with “created” because that also sounds like the artisan who made a specific one. Should it be renamed to “invented” or something?
+1 for “invented” (“created” is definitely confusing with instrument maker)
Invent does sound better. Oxford says: “Create or design (something that has not existed before); be the originator of.”
Which sounds exactly like what we want here.
Edit: “Developed” might be a candidate too.
Renamed to “invented”
want to chip in with that “invented” sounds better for me too (my apologies as I’m not native english so I used a less better word in my bugpost : ) thank you everyone! \o/)